A flip chip is a small semiconductor chip (chip) having terminations all on one side in the form of solder pads or bump contacts. Typically, the surface of the chip has been passivated or otherwise treated. The flip chip derives its name from the practice of flipping, or turning, the chip over after manufacture, prior to attaching the chip to a matching substrate.
Electronic packages, which include chips (including flip chips), usually require a cover, cap, or encapsulant of some type over the chip to protect it, and to provide a large flat surface for pick-and-place operations. However, any cover, cap, or encapsulant above the chip increases the thermal resistance path to an ambient environment and, hence, the operational temperature of the chip.
Various means have been used to mitigate the effects of such covers or caps. One approach has been to use a capped chip with a thin layer of a thermally conductive grease between the chip and the cap. However, it has been found that during thermal cycling, the grease has a tendency to be pumped, or displaced from the interface between the chip and the cap, thus increasing the thermal resistance of the interface. Pumping, or grease pumping as it is commonly referred, results as the gap between the chip and the cap increases and decreases, eventually forcing enough grease out of the gap, and forming a critical number of voids, to hinder thermal performance. This change in the gap thickness results as the substrate, to which the chip and cap are attached, and the cap bend to differing radii of curvature, due to the coefficient of thermal expansion (CTE) mismatch of the cap and substrate under thermomechanical loads. The stiff (nearly solid) nature of the grease contributes to the formation and coalescence of voids under this pumping action. A critical magnitude of tensile/compressive grease strain exists which results in maximum allowed thermal degradation. Another approach has been to attach a cap to the chip with an adhesive. This presents an opportunity to “balance” the package by matching the expansion and stiffness of the cap to that of the substrate so that the electronic package remains nearly flat at all temperatures. This approach is not, however, without problems. The CTE mismatch between the chip and the cap produces interfacial stresses between the adhesive and both the surface of the cap and the top surface of the chip. These stresses can cause delamination during the manufacturing process or during thermal cycling (chip operation in the field) near the corners and edges of the chip. Once delamination begins, crack propagation may ultimately lead to a defective product and/or electrical failure in the field. This is a major problem with capped chips particularly with flip chips on circuitized substrates of the organic type, such as epoxy laminate types.
The present invention is directed at overcoming the problems set forth above. It is desirable to have an electronic package and method to make the package that provides low thermal resistance, minimizes interfacial stresses, and is economical to manufacture. It is also desirable to have such an electronic package and method of manufacture that uses conventional assembly techniques. It is further desirable to have such an electronic package that does not require the presence of a thermally conductive grease between the chip and a thermally conducting member such as a cover or cap. Electronic packages produced by the method of this invention will have superior thermal performance and have much improved operational field life.